This invention relates generally to rotating dynamoelectric machines, and it relates more particularly to improvements in the electrical insulation of an armature coil for the rotor of such a machine.
In large, relatively high horsepower direct current (d-c) dynamoelectric machines such as locomotive traction motors, the armature comprises a rotatable, cylindrical core of ferromagnetic laminations having a plurality of slots in its periphery for receiving armature coils that are electrically connected to an external circuit via a rotating commutator and cooperating stationary brushes. Each armature coil has multiple individual turns. It typically is formed by covering each of a plurality of long, thin copper bars with suitable dielectric material to provide turn-to-turn insulation, binding a set of eight (more or less) of these bars in parallel and bending the set into a generally rectilinear winding whose opposite sides are straight and parallel, and then covering the winding with a sheath of suitable dielectric material so as electrically to insulate the bundle of juxtaposed bars from the exposed edges of the core laminations that define the sidewalls and bottoms of the slots in which the straight sides of the coil are placed and that are at ground potential. There are several known techniques for applying the turn-to-ground insulating sheath on a multiple turn armature coil. One is to wrap each of the straight sides of the bundle of copper bars (i.e., the slot section of the coil) in multiple layers of a thin sheet of dielectric material; see prior art U.S. Pat. Nos. 2,675,421 and 2,697,055. In a second known method, each slot section is encased between a pair of complementary, pre-formed, relatively inflexible channel-shaped members of dielectric material; such members are easy to manufacture and assemble but provide relatively poor creepage to ground. A third method is to spiral wrap the sides of the coil in insulating tape. The third method can be used either alone, in combination with the first method (see U.S. Pat. No. 3,662,199), or in combination with the second method.
Whatever method is used, the turn-to-ground insulation of an armature coil needs to have sufficient dielectric strength, thickness, and integrity to prevent electrical breakdown (i.e. short circuits) from any turn in the coil to ground under all possible environmental conditions which, for a locomotive traction motor, include constant vibration, frequent mechanical shocks, occasional electrical overloads, a wide range of ambient temperatures, and an atmosphere that can be very wet and/or dirty. And the desired insulating properties need to be maintained, without appreciable deterioration, as the machine ages and in spite of cyclic changes, due to temperature excursions, in the length of each slot section of the coil relative to the longitudinal dimension of the sidewalls of the associated slot. (As is well known, each time the average magnitude of armature current is increased to its full-load rating, the heating effect of this current will cause the copper bars in the coil to expand, and the amount of such expansion differs from that of the laminated core which initially is cooler than the coil and which in any event has a different coefficient of thermal expansion.)
Good heat transfer is another generally desirable characteristic of the insulating system. This characteristic is particularly significant in traction motors where the goal is to obtain more output torque per unit of weight. To help attain this goal, any one or combination of the following possible changes to the armature of the motor is desirable: (1) increase the cross-sectional area of the copper bars in each armature coil for a slot of given size, thereby allowing the coil to conduct more current without increasing current density; (2) increase the current density (and consequently the heat generated) in the bars; (3) decrease the depth of each slot so that bars of given cross-sectional area are located closer to the surface of the core and hence closer to the field poles of the machine. But none of these changes can be achieved without reducing the thickness of the turn-to-ground insulation of the armature coil. The thinner the outer sheath of insulation on the armature coil, the more space for the copper bars inside the sheath and the better the transfer of heat from the bars to the rotor core. By thus reducing the generation of heat and/or promoting its dissipation, the armature coil can carry more current (and the motor can therefore develop more torque) without exceeding a given maximum safe temperature rise.
An insulating material that is particularly advantageous for traction motor applications is known generically as Type H polyimide film. An FEP-fluorocarbon resin coated form of such film is manufactured and sold by Dupont Company under its trademark "Kapton." Thin gauge Kapton insulation is very flexible, has a relatively high dielectric strength (typically at least 3,000 to 4,000 volts per mil), and remains physically and electrically stable at elevated temperatures. The coating of FEP-fluorocarbon resin (popularly known by the Dupont trademark "Teflon") provides a very smooth, heat-sealable surface on the base of the polyimide film. This also improves the chemical resistance of the film and reduces the rate of moisture permeability and of oxidative decomposition. Such composite material has been heretofore used successfully to insulate rectangular motor magnet wire and to insulate the field coils of locomotive traction motors (see U.S. Pat. No. 4,376,904-Horrigan). Precision motors have heretofore used slot liners made of H film. For more information about Kapton and its typical prior uses, see the paper by D. H. Berkebile and D. L. Stevenson titled "The Use of `Kapton` Polyimide Film in Aerospace Applications" published in 1982 by the Society of Automotive Engineers at pages 3562-68 of its Conference Record of an SAE meeting on Oct. 5-8, 1981 (preprint No. 811091).